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Lime–soda softening in water treatment Which constituent is primarily removed from water using the lime–soda process during conventional softening?

Difficulty: Easy

Correct Answer: Permanent hardness

Explanation:


Introduction / Context:
Water softening protects boilers, heat exchangers, and distribution systems from scale caused by hardness ions. The lime–soda process is a classic chemical softening method in municipal and industrial water treatment.



Given Data / Assumptions:

  • Lime–soda uses lime (Ca(OH)2) and soda ash (Na2CO3).
  • Hardness arises mainly from Ca2+ and Mg2+ salts.
  • We must identify which type of hardness is the principal target.


Concept / Approach:
Lime alone removes temporary (carbonate) hardness by precipitating CaCO3 and Mg(OH)2. Adding soda ash (the soda in lime–soda) enables removal of non-carbonate (permanent) hardness by converting Ca2+ to CaCO3 via carbonate addition. Thus the hallmark of the lime–soda process is its ability to remove permanent hardness efficiently in addition to reducing temporary hardness.



Step-by-Step Solution:

Temporary hardness: removed by lime; Mg2+ also precipitated as Mg(OH)2 with sufficient pH.Permanent hardness: requires carbonate addition (soda ash) to precipitate CaCO3 from CaSO4/CaCl2, etc.Key distinction: the “soda” component specifically targets permanent hardness.Hence, the primary removal associated with lime–soda is permanent hardness.


Verification / Alternative check:
Standard design texts show stoichiometric soda dosing proportional to non-carbonate hardness; jar tests and residual hardness calculations confirm effectiveness.



Why Other Options Are Wrong:

Foul smell and taste: Addressed by aeration, activated carbon, or oxidation—not by lime–soda chemistry.Iron and manganese: Typically removed via oxidation and filtration; softening may incidentally remove some but is not the primary method.Temporary hardness: Reduced by lime, but the question emphasizes the lime–soda process, whose distinguishing feature is permanent hardness removal.


Common Pitfalls:
Assuming softening targets only carbonate hardness; the soda step is specifically for non-carbonate (permanent) hardness.



Final Answer:
Permanent hardness

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